Specifically for sharing models between spaces, we have a feature in beta using our graphql API.
For sharing content entries between spaces, you can currently do that by downloading your content, right clicking your content in the editor and select “download content as JSON.” Then you can navigate to a page in your other space, right click in the editing window, and select “upload builder.json file.” These steps are outlined here as well.
Besides multiple spaces, there are a couple of other options for managing multiple environments in Builder.